The invention relates to camshaft adjustment device for the adjustment of a phase position of a camshaft relative to a crankshaft of an internal combustion engine which includes a hydraulic system for supplying hydraulic operating fluid to the operating chambers of the camshaft adjustment device.
Adjustment devices for controlling the phase position of a camshaft relative to a crankshaft of an internal combustion engine often employ a hydraulic operating mechanism including vanes mounted on a first body which is rotatable with the camshaft and a second body which receives the first body so as to be rotatable therein and is provided with counter vanes. The two bodies form between the vanes operating chambers to which hydraulic operating fluid can be supplied. An adjustment of the hydraulic cam adjuster results normally in a pressure drop in the oil supply to the camshaft adjuster. Consequently, at low engine speed and hot engine, operation of the camshaft adjuster may result in an undesirable oil pressure drop below the required minimum pressure of the engine. As a result, for example, the camshaft bearings may wear excessively the engine life may be shortened.
In order to avoid such an oil pressure drop, a larger oil pump may for example be used. This however increases the power consumption of accessory device and consequently the fuel consumption for a motor vehicle with the same driving performance. Alternatively, pressure stores may be used which, however, require a larger space which is generally not available. Novel camshaft adjuster includes a hydraulic circuit via which the actuation of the angular adjustment occurs passively by a camshaft torque changing over a shaft rotation one or several times. Such camshaft adjusts provide on the basis of inertia however only a passive “adjustment function” up to a certain maximum limit speed above which no adjustment is possible. The necessary adjustment speed however becomes continuously smaller with increasing rotational speed already below this limit speed.
For some time now, embodiments of adjusters are known which utilize the varying torque of the camshaft for enhancing the adjustment by hydraulic fluid or oil pressure. These adjusters require as additional design component a check valve which is arranged in the oil supply path ahead of a proportional valve which is needed for the control. By this check valve, a back flow of oil into the engine oil circuit caused by the counteraction of the camshaft torque against the desired adjustment direction is prevented. If the effective direction of the camshaft torque corresponds to the desired adjustment direction the camshaft adjuster acts like a pump and sucks oil out of the engine oil circuit into the adjuster. Particularly with high adjustment speeds, this may result in a drop of the engine oil pressure which, at low engine speeds and hot engine oil, may drop below the limit values.
Furthermore, the varying torque of the adjuster may be used as driving means. To this end, the outlet of the chamber disposed in adjustment direction must be connected with the supply to the counter chamber. To ensure that only the part of the camshaft torque is used which acts in the adjustment direction, the flow direction of the oil must be determined by the control arrangement. This can be ensured by the check valve. An adjuster operating with this operating principle requires from the engine oil circuit the lubricant only once for the filling of the adjustment chambers, as well as some oil for the continuous replacement of leakage oil so that no oil pressure drop occurs during the adjustment procedure. However with this adjustment principle, the adjustment dynamics drops continuously with increasing rotational speeds. Beginning at a certain maximum speed an adjustment is no longer possible.
A hydraulic camshaft adjuster based on a vane structure is known from EP 4 073 830 A1.
In hot arrangement, the oil needed for the adjustment of the camshaft is taken from the camshaft via a control valve disposed at a hydraulically suitable location in a central mounting bolt. The adjuster can be operated also at relatively low oil pressures because the pressure losses are minimized by short control lines, although a certain pressure drop is unavoidable.
U.S. Pat. No. 5,107,804 discloses a passive camshaft adjuster which, in addition to a check valve in the oil supply line, includes a check valve in the oil supply from the control valve to each hydraulic operating chamber of the adjuster. This increases the number of components and, with more than two operating chambers, requires the provision of expensive oil channels and increases the manufacturing expenses. An adjustment at higher engine speeds is possible with this circuit only in a limited way because of the mechanical inertia.
DE 42 29 201 C2 discloses a camshaft adjuster whose hydraulic circuit includes a control valve by which the oil return flow is conducted from the camshaft retarding operating chambers selectively to the operating chambers which advance the camshaft or to the tank. In this way, the adjuster can be switched by electromagnetic actuation from a “passive adjustment” by means of camshaft torques to an active adjustment by means of oil pressure. However, the additional valve with a separate electrical switching operation is disadvantageous as the adjuster and the respective motor control becomes more complicated and expensive.
Furthermore, EP 1 221 540 A1 discloses a camshaft adjuster with reduced control expenditures. This reduction is achieved by the replacement of magnetic actuation by an oil pressure or centrifugal force actuation of the switch-over valve for the particular operation. The hydraulic actuation however requires a larger space because of the presence of a control valve, check valves and a switch-over valve in the camshaft adjuster.
It is the object of the present invention to provide a camshaft adjustment device for the phase adjustment of a camshaft relative to a crankshaft of an internal combustion engine which provides for optimal adjustment functions also at low oil pressure levels and, which, at the same time, is of compact design.